A. Field of the Invention
The present invention generally relates to an adiabatic scanning calorimeter for simultaneous measurements of the temperature dependence of heat capacity and enthalpy of liquids and solids and phase transitions therein. Moreover, the invention allows for an accurate separation between pretransitional enthalpy variations and true latent heats at first-order or weakly first-order phase transitions. In addition, the invention relates to calorimeters for controlling temperature differences and heat fluxes in different modes of operation.
B. Description of the Related Art
Measurements of the heat capacity and enthalpy changes play an important role in monitoring the energy content of condensed matter systems. As such calorimetry is an indispensable technique for many scientific fields. Depending on the application envisioned several different technical approaches with varying degrees of accuracy and precision have been developed. Over wide temperature ranges generally the classical Nernst heat pulse method is used.1 During the last 50 years several new approaches, supported to a large extent by novel developments in electronic measurements instrumentation, have emerged, e.g. differential scanning calorimetry (DSC),2,3 scanning transitiometry4-6 and modulation techniques like ac calorimetry7,8, the 3ω method9 and more recently photoacoustic and photopyroelectric techniques,10 Peltier ac and Peltier tip calorimetry,11,12 Peltier heat-flow and modulated bath ac calorimetry.13,14 
A novel development beyond classical adiabatic heat pulse calorimetry, took place at the end of the 1960s when Australian scientists15-17 imposed a very slow constant heating (or cooling) rate on the thermal shield (in a classical type adiabatic calorimeter) surrounding the sample cell and the cell was forced to follow with the same rate. By measuring the imposed rate and the power applied (heating) to or extracted (cooling) from the cell, the heat capacity C is readily obtained from
                              C          =                                    T              ⁢                                                ⅆ                  S                                                  ⅆ                  T                                                      =                                                            ⅆ                  Q                                                  ⅆ                  T                                            =                                                                                          ⅆ                      Q                                        /                                          ⅆ                      t                                                                                                  ⅆ                      T                                        /                                          ⅆ                      t                                                                      =                                  P                  /                                      T                    .                                                                                      ,                            (        1        )            with S the entropy, T the temperature, dQ the supplied heat, t the time, P the supplied power and {dot over (T)} the temperature scanning rate. If one considers the shield (forced to change its temperature at constant {dot over (T)}) as the reference ‘sample’, the setup is conceptually similar to the (power compensated) differential scanning calorimeter. There are, however, basic differences in design principles and area of applications. The DSC is a very useful for many (material science) applications when the (total) energy change of a transition is of greater interest than the detailed form of the specific heat or enthalpy curve (near phase transitions). A commercial DSC (or modulated DSC) generally does not yield accurate absolute values of specific heat and by using high scanning rates (typically above 0.2 Ks−1 to have a reasonable sensitivity) quite often operates out of thermodynamic equilibrium, in particular near fluctuations dominated phase transitions. Moreover, with DSC it is often not possible to discriminate between second-order (continuous) phase transitions and (weakly) first-order ones.18 Several of the limitations of DSCs have been eliminated in scanning transitiometry by imposing very slow constant scanning rates in a high precision differential concept4-6. However, imposing constant rates in this approach remains a basic problem for high-resolution work at and near (weakly) first-order transitions. Buckingham and coworkers called their apparatus a high precision scanning ratio calorimeter (for use near phase transitions).17 In order to cope with the critical slowing down near the investigated liquid-gas critical point, they imposed constant scanning rates as low as 10−6 Ks−1. In the mid 1970s a group at the Catholic University of Leuven (Belgium) built a four stage scanning calorimeter to measure with high resolution the heat capacity (at constant pressure) near critical (consolute) points of binary and ternary liquid mixtures.19-22 The construction of that calorimeter was such that in addition to different scanning modes it could also be used as a classical step calorimeter. It was also realized that near phase transitions and critical points it would be much easier to cope with the critical slowing down and the large increase of the heat capacity and possible latent heats, by imposing a constant heating or cooling power to the sample and determine the rate instead of imposing a constant heating or cooling rate as was done before, i.e. keeping P constant and not {dot over (T)} in Equation (1).20,21 In fact, this change in operation mode is essential for the proper investigation of (weakly) first-order phase transtions.23,24 It is quite straightforward to show that the direct experimental results of the (constant) power P and the temperature T(t) of the sample as function of the time t (since the start of the run at T(ts)) yields the temperature dependence of the enthalpy (including a value of the latent heat when present) byH(T)=H(Ts)+P(t−ts)  (2)
Around that time, calorimeters similar to the Leuven adiabatic scanning type calorimeter were developed by other groups as well. In 1980 Würz and Grubić25 described a three stages adiabatic calorimeter of the scanning ratio type and did measurements at constant scanning rates of 128.8 μKs−1 and 6.98 μKs−1 near a liquid-liquid critical point, Junod25 described a setup with a continuous adiabatic (scanning) method for the graphical recording of the heat capacity of solids over the temperature range between 80 K to 320 K at moderate to fast scanning rates (typically around 10 mKs−1). A microcomputer controlled ASC type apparatus for solid samples was described in a paper of 1981.27 After the introduction of adiabatic scanning calorimetry (ASC) for first-order and second-order phase transition studies in liquid crystals23,24 it was also used for liquid crystal studies by Anisimov and coworkers.28 Bessergenev et al. used different ASC modes of operation to study first-order and second-order transitions in rear earth metals.29 Lysek et al. described a scanning ratio calorimeter (at rates of about 1 mKs−1) for use in adsorption studies.30 An ASC technique similar to the Leuven one was used by Sirota to study phase transitions and super cooling of normal alkanes.31,32 Schnelle and Gmelin introduced a high resolution ASC for small (solid) samples.33 Moon and Yeong proposed, in 1996, a so-called rate-scanning modified adiabatic calorimeter (MAC) (with scanning rates between 0.2 mKs−1 and 30 mKs−1).34,35 However, their setup is operationally the same as the previously well established standard ASCs as used by several other groups. An ASC similar to the Leuven one for the study of liquid-liquid critical points was built by Jacobs and collaborators.36 
An important requirement, of a high-resolution adiabatic calorimeter operating in the heating mode is the equality (better than a mK) of the temperatures of the sample and the surrounding thermal shield. For operations in the cooling mode a constant preset temperature difference between the sample and the shield has to be maintained within the same stability limits. This is presently achieved using thermistors as highly sensitive resistance thermometers placed on the sample and on the shield. Before these sensors can be used, time consuming extensive calibrations (against reference thermometers) have to be executed. Moreover, the temperature coefficients of the resistance of two thermistors do never perfectly match. Via hardware adaptations in the measuring circuits23 or in software modifications of the calibration curves, one can partly correct for it. The present invention eliminates these problems completely by inserting between the sample and the shield a very sensitive (of the order of 0.1V/K) semi-conductor materials based Peltier element (PE), either a Peltier cooler or Peltier thermogenerator, which are commercially available. The μK sensitivity of the PE for temperature differences allows in combination with proper servo systems (hardware or software) to maintain almost perfect equality of the sample and shield temperatures in the heating mode. For the cooling mode a preset temperature difference between sample and shield can be kept constant with equal resolution.